An example of a structure of a conventional ridge-stripe semiconductor laser device is shown in FIG. 9. Here, a GaAS/AlGaAs ridge-stripe semiconductor laser device is explained as a representative example.
In this semiconductor laser device, on a n-GaAS substrate 101, a n-AlGaAs cladding layer 103, a quantum-well structure active layer 104, a p-AlGaAs cladding layer 105, a p-AlGaAs semiconductor layer 107, a p-GaAs cap layer 109 and a p-GaAs contact layer 111 are formed in order. The p-AlGaAs semiconducotor layer 107 and the p-GaAs cap layer 109 are shaped into a ridge, and a high-resistive or n-type current blocking layer 110 is formed on both sides of the ridge.
In addition, on the p-AlGaAs cladding layer 105 is formed an etching-stopper layer 106 comprising GaAs.
A process flow of forming a conventional ridge is shown in FIG. 10. In this figure, the n-GaAS substrate 101, the n-AlGaAs cladding layer 103 and the quantum-well structure active layer 104 are not shown.
First, a common photoresist film is formed on a surface of a wafer on which the formation of the p-GaAs cap layer 109 has been finished, and an etching mask 113 is formed against etching for forming a ridge pattern using a general photolithography technique (FIG. 10a).
Next, the wafer is etched to the middle of the p-AlGaAs semiconductor layer 107 using a sulfuric acid/hydroperoxide etchant (FIG. 10b) and, subsequently, the wafer is etched to the etch-stopper layer 106 with hydrofuloric acid. Since hydrofuloric acid dissolves AlGaAs but GaAs, when the remaining p-AlGaAs semiconductor layer 107 is etched with hydrofuloric acid, etching will stop at the etch-stopper layer 106.
Further, etching is continued to adjust a width of a lower part of the ridge to a predetermined width (FIG. 10c).
After that, the photoresist film, i.e., the etching mask 113 is peeled by an organic solvent to complete the formation of the ridge stripe (FIG. 10d).
On the wafer on which ridge formation had been done in this way, a current blocking layer 110 was formed by using the MOVPE process (FIG. 10e).
Then, a photolithography was performed using a photoresist film in order to remove an unnecessary part 110a of the current blocking layer 110 growing on the ridge stripe which is a current constriction part, and the surface of wafer other than the unnecessary part 110a was covered with an etching mask 114 (FIG. 10f).
After that, wet-etching was carried out to remove the unnecessary part 110a on the ridge stripe (FIG. 10g) and, subsequently, the etching mask 114 was removed with an organic solvent (FIG. 10h).
After washing the wafer surface, a p-GaAs contact layer 111 was formed by using the MOVPE process again to complete the ridge stripe semiconductor laser wafer (FIG. 10i).
However, when ridge stripes are shaped only by wet-etching as described above, the following problems occur.
In the case of wet-etching, since etching proceeds isotropically, in order to obtain a predetermined width, it is required that a ridge pattern width of the etching mask 9 is set to a width wider than the predetermined edge width with considering an amount of side etching.
In addition, due to the side etching, the ridge height is limited and it is difficult to control the shape of the ridge strictly. Although lower ridge is convenient because production is easier, when the ridge is too low, a light diffused from the active layer is absorbed by the p-GaAs cap layer, resulting in deterioration in laser device properties, such as increase in a threshold current, reduction in a differential efficiency and the like. Therefore, the ridge height should be 1.5 μm or larger and, furthermore, in a high-output laser in which the diffusion of the light from the active layer is large, formation of a higher ridge is desired.
Further, since the p-AlGaAs semiconductor layer 107 is etched with hydrofuloric acid, only the sidewalls of the p-AlGaAs semiconductor layer 107 is selectively etched without etching the GaAs cap layer 109. Consequently, side etching of the p-AlGaAs semiconductor layer 107 proceeds around the interface between the p-GaAs cap layer 109 and the p-AlGaAs semiconductor layer 107.
As a result, since the width of the p-GaAs cap layer 109 becomes wider than a predetermined ridge width by setting the width of the ridge pattern of the etching mask to a width wider than the predetermined ridge width, and since the width of the p-AlGaAs semiconductor layer 107 just under the p-GaAs cap layer 109 becomes narrower than the predetermined ridge width due to side etching, overhanging parts are formed at an upper part of the ridge, and the ridge stripe has a shape like a mushroom with a wide cap.
When a n-current blocking layer 110 is grown on a wafer having such a ridge shape by an epitaxial growth technique, such as a metal organic vapor phase epitaxial (MOVPE) process, an epitaxial growth does not completely proceed just under the overhanging parts of the p-GaAs cap layer 109, resulting in the formation of cavities 112 (FIG. 10e). These cavities never disappear in the following steps (FIGS. 10f˜i) and remains in the completed laser device.
The existence of such cavities and fluctuation in the ridge width lead to reduction in differential efficiency of the laser device properties, increase in a threshold/working current, occurrence of kink and the like to deteriorate laser device properties.
In JP-A 6-268317 discloses a process for suppressing the formation of cavities under a dielectric mask (SiO2) for wet-etching in a step of shaping striped ridges by removing the dielectric mask after shaping a ridge by wet-etching to make its width narrower than a predetermined width of the ridge stripe. However, although this process is useful for suppressing the formation of cavities at the side of the ridge, this requires etching treatment for adjusting the size of the dielectric mask and, accuracy of the etching control is not high.
An example of a structure of a conventional high-output ridge-stripe semiconductor laser device is shown in FIG. 11. Here, a GaAS/GAInP/AlGaAs ridge-stripe semiconductor laser device is explained as a representative example.
In this semiconductor laser device, on a n-GaAS substrate 101, a n-GaInP intermediate layer 102, a n-AlGaAs cladding layer 103, a quantum-well structure active layer 104, a p-AlGaInP cladding layer 105, a p-AlGaAs semiconductor layer 107, a p-GaInP intermediate layer 108, a p-GaAs cap layer 109 and a p-GaAs contact layer 111 are formed in order. The p-AlGaInP semiconductor layer 107, the p-GaInP intermediate layer 108 and the p-GaAs cap layer 109 are shaped into a ridge, and a high-resistive or n-type current blocking layer 110 is formed on both sides of the ridge. This current blocking layer 110 is formed with AlInP for high-output use.
In addition, on the p-AlGaInP cladding layer 105 is formed an etching-stopper layer 106 comprising GaAs.
A process flow of forming a conventional ridge is shown in FIG. 12. In this figure, the n-GaAS substrate 101, the n-GaInP intermediate layer 102, the n-AlGaInP cladding layer 103 and the quantum-well structure active layer 104 are not shown.
First, an aluminium oxide (Al2O3) film is formed on a surface of a wafer on which the formation of the p-GaAs cap layer 109 has been finished by a molecular beam epitaxial (MBE) process, and an Al2O3 etching mask 113 is formed against etching for forming a ridge pattern using a general photolithography technique and by wet-etching (FIG. 12a).
Next, the p-GaAs cap layer 109 is etched using a sulfuric acid/hydroperoxide etchant and, subsequently, the p-AlGaInP semiconductor layer 107 is etched halfway using a bromine/phosphoric acid etchant and, lastly, the remaining semiconductor layer 107 is etched to the etch-stopper layer 106 with a hot phosphoric acid etchant to complete the formation of the ridge stripe (FIG. 12c).
On a wafer on which ridge formation had been done in this way, a current blocking layer 110 was formed by using the MBE process (FIG. 12d).
Then, a photolithography was performed using a photoresist film in order to remove an unnecessary part 110a of the current blocking layer 110 growing on the ridge stripe which is current constriction part, and the surface of wafer other than the unnecessary part 110a was covered with an etching mask 114 (FIG. 12e).
After that, wet-etching was carried out to remove the unnecessary part 110a on the ridge strip (FIG. 12f) and, subsequently, the etching mask 114 was removed by UV ozone ashing (FIG. 12g).
After washing the wafer surface, a p-GaAs contact layer 111 was formed by using the MBE process again to complete the ridge stripe semiconductor laser wafer (FIG. 12h).
However, when ridge stripes are shaped only by wet-etching gas described above, the following problems occur, and it is difficult to make a high-output laser device.
In a real-guide structure device used for a high-output laser light, it is required to make the ridge height higher and to make the ridge width narrower. Conventionally, ridge stripes have been shaped only by wet-etching. However, since, in the conventional method, a generally vertical ridge is not obtained and its shape becomes a trapezoid shape, it is very difficult to make a ridge with the above mentioned shape.
In addition, since for high-output use, the composition of the n-current blocking layer is changed from GaAs to AlInP, which has higher etch rate against phosphoric acid, the current blocking layer 110 growing on the ridge is etched with a hot phosphoric acid etchant (FIG. 12f), leading to insufficient formation of the current blocking layer. As a result, the obtained laser device does not properly function.